This application relates generally to disc drives and more particularly to an apparatus and method for detecting errors during a write operation in a disc drive.
One of the key objectives of disc drive designers is to maximize the reliability of data in disc drives. This objective is often constrained by the performance requirement of disc drives. That is, increasing the reliability of data often reduces performance of a disc drive. For example, when data is written to the media in a disc drive, the success of a write operation, i.e. whether the media accepted the data, is unknown. One way of ensuring the success of a write operation is by reading the recorded data after each write operation. If there is a data error or data loss, then a re-write operation will be performed. This technique requires all written data to be read which ensures the reliability of the data. However, such technique substantially increases the operation time thereby severely degrading the performance of the disc drive.
A skip write is a write error caused when an object, such as a particle of material, e.g. dust, gets temporarily lodged between the head and the media, causing the head to move away from the media surface. When this happens, the level of magnetic saturation in the media is reduced dramatically, and when the data is subsequently retrieved, errors are present.
The existing techniques for detecting a skip write in a write operation have involved verifying written data via a reading process as described above or a reading process as described in U.S. Pat. No. 5,588,007. Once an error is detected, a re-write operation is performed.
Accordingly there is a need for an efficient technique to detect a skip write in a write operation.
Against this backdrop embodiments of the present invention have been developed. Embodiments of the present invention include a skip write detection apparatus and method that detects a sudden change of back emf in a read/write head of a disc drive during a write operation which solve the above-mentioned problem.
One embodiment of the present invention, an apparatus for detecting a skip write in a disc drive, includes a sample module to sample write signals from a write element of a read/write head and a detect module to detect occurrence of a sudden change in back emf of the sampled write signals. The detect module includes an amplifier which amplifies the sampled write signals, a filter which filters out acceptable back emf effects of the sampled write signals, and a comparator which compares an amplitude of the back emf of the sampled write signals passing through the filter to a predetermined value. If a skip write occurs, the sudden change in the back emf is indicated by an output of the comparator. The write signals may be voltage signals or current signals.
The output of the filter is preferably connected to an A/D converter to convert the analog write signals to digital write signals before the signals are sent to the comparator.
A threshold module is also connected to the comparator. The threshold module provides the predetermined value when no skip write happens. Further, the predetermined value may be varied by a media zone factor depending on the media zone that the read/write head flies over. The media zone factor is represented by a value preferably stored in a media zone lookup table. In one embodiment, the threshold module includes a programmable register for storing the predetermined value. The predetermined value is sent to one of the inputs of the comparator for detecting the occurrence of a sudden change in the back emf.
Another embodiment of the present invention is a method of detecting a skip write in a disc drive that includes steps of monitoring write signals having back emf, and detecting occurrence of a sudden change in the back emf. Once the sudden change in the back emf is detected, which likely corresponds to the detection of the skip write event, a re-write operation is performed.
The step of monitoring preferably further includes a step of sampling the write signals. The step of detecting preferably further includes steps of amplifying the sampled write signals, filtering out acceptable back emf effects of the sampled write signals, and comparing an amplitude of the back emf to a predetermined value. If a skip write occurs, the sudden change in the back emf is indicated by an output of a comparator. The write signals may be voltage signals or current signals and the method may include a step of converting the analog write signals to digital write signals.
Steps providing a predetermined value when no skip write happens, i.e., when the head is flying at a nominal fly height, and adjusting the predetermined value by a media zone factor depending on a radial media zone that the read/write head flies over may also be included. The media zone factor may be a value preferably stored in a media zone lookup table. The adjusted predetermined value is sent to the comparator for comparing it to the amplitude of the back emf of the sampled write signals.
If the read/write head is moved away from the media during a write operation, i.e. a skip write occurs, the amplitude of the back emf suddenly changes, e.g. in proportion to the loss of magnetic coupling between the read/write head and the media. Such sudden change is detected immediately by embodiments of the present invention. The present write operation can be abandoned and a re-write operation can be initiated immediately. Thus, efficient detection of skip write in a write operation of a disc drive is provided in accordance with the embodiments of the present invention, and the skip write is detected without undergoing a reading verification process.